During your childhood, have you ever mixed dish soap with oil, vinegar with coffee, shampoo with coke? If not, you have no idea what you are missing! As a kid I was always mixing different liquid products (e.g. detergents in my home, dressings at restaurants) pretending to be a chemist. Mixing things and waiting for the result was always my passion. So, the picture below is really exciting for me, as you can figure out. Different solutions, different colours, but all with the same metal: Copper (Cu).
These solutions are the result of copper oxides (CuO and Cu2O) dissolution in different deep eutectic solvents (what are these solvents? If you don’t know, check my previous article!) Ethaline 200, Reline 200, Maline 100, Lactiline 100, Oxaline 100, Ethaline with 500 mM oxalic acid, Ethaline with 500 mM citric acid.Copper ions can have different colours depending on the valence state of the ion in the solution and the complexes made upon dissolution. For example, an experiment of professor Koen Binnemans at KU Leuven  showed that Cu+2 appears in a range of colours from pale blue, over green and greenish yellow, to yellow depending on the concentration of water or choline chloride in the solution or in other terms depending on the coordination of copper in the solution or in the simpler words of what atoms are surrounding Cu in the solution. Each anion of these complexes interacts with Cu in a different manner, resulting in what we see, different colours.
But how can we know, which species we have produced inside the solution. Yes, the colour gives us a first hint but how can we prove the existence of specific species of metals? There is a plethora of different spectroscopic techniques that analytic chemists and metallurgists are using with the basics to be UV – Visible, Infrared, Raman and NMR spectroscopy. I guess you are wondering, what is spectroscopy in the first place? Spectroscopy is the study of the interaction between matter and electromagnetic radiation. 
For my PhD project, I am using UV – Vis spectroscopy almost every day, as it is a very easy and straightforward technique to gain information about the complexes made in the solution by my targeted metal. With this technique, radiation belonging to the Ultraviolet (UV) and Visible (Vis) frequencies of the electromagnetic spectrum (Figure 2), is interacting with the different coloured solutions. Depending on the existent complexes in the solution, different wavelengths of radiation are absorbed.  For example, in Figure 3 a typical UV-Vis absorption graph is shown. The peak at 406 nm, it is well known to belong to CuCl42- complexes.  Therefore, I know that copper made complexes with chlorides present in the solution. In the case of UV-Vis I am not able to characterise my species in detail, I might use EXAFS (X – Ray absorption fine structure, you don’t want to know, trust me).
A useful tip for you to know: When white light passes through or is reflected by a coloured substance, a characteristic portion of the wavelength is absorbed. Now, check the colouring wheel in the Figure 4. The colours that are across each other are called complementary colours. So, when a coloured substance absorbs the orange light at the region of wavelengths between 600 – 640 nm then we see blue colour (because orange and blue are at the opposite side) and vice versa. Cool, right? Now you can explain why we see specific colours.